Recently, mobile devices such as cell-phones (and in particular smart-phones) have started to incorporate a front-facing projection-and-imaging system that is used for applications such as face identification and face tracking. Often, such systems are based on a structured light (SL) projector that may operate in the near-infra-red (NIR) range and which projects a pattern onto a user's face. Structured light is a process of known pattern projection (e.g. dots or lines) onto an object or scene. Pattern imaging with a camera (e.g. a dedicated and pre-calibrated camera) allows depth calculation of the object or the scene.
In this disclosure, NIR refers to the wavelength range of 780 nm-1120 nm, which is invisible to the human eye. A NIR-sensitive camera images the face with the pattern projected on it. A processor can then calculate depth from the relative position of pattern elements projected on the face. This depth information is then used for many applications, like 3D modeling of the face, security face identification (such as unlocking the mobile device, authorizing payments, etc.), augmented reality applications, camera effects (such as bokeh), avatar animation, etc.
In some cases, the SL projector component comprises an array of vertical cavity surface emitting laser (VCSEL) elements that are replicated using a diffractive optical element (DOE) (e.g. a Dammann diffraction grating) and then imaged onto a scene. Each VCSEL creates a separate dot of NIR light. The spatial arrangement of the VCSELs in the array and the replicating DOE determines how the dots are imaged onto the scene.
Projecting a pattern in the NIR range on a user's face can potentially be harmful to the eyes. The retina is sensitive to light in the NIR range. Research has shown that prolonged exposure to it, beyond a certain intensity threshold, may cause damage to the retina in the form of IR cataracts and flash burns due to a rise in temperature. Therefore, eye safety regulations normally limit the intensity and duration of SL pattern projection (see, e.g. regulation IEC/EN 62471 and others).
It is therefore beneficial to have a front-facing structured light system that can minimize or eliminate projection of NIR light onto the eyes and increase. Such a system can also increase the light intensity provided by a SL projector while keeping eye safety regulations.